A coil is an important part in a magnetic resonance imaging system and before it leaves a factory or is put into use it generally needs to undergo various tests. Taking a gradient coil in a magnetic resonance imaging system as an example, besides needing to undergo routine resistance, inductance and high-voltage tests, it also needs to undergo a test for gradient magnetic field distribution.
As to a magnetic field which is close to the magnetic field strength of the earth, if a test is carried out directly, the interference will be quite significant so the accuracy will not be high. Since the magnetic field generated by a gradient coil is relatively weak and is close to the magnetic field strength of the earth, it is not feasible to supply a direct current to a gradient coil for a direct test of the magnetic field in air.
Nowadays the procedure for testing the gradient field before delivery is simply omitted by many manufacturers, and only routine resistance, inductance and high-voltage tests are performed. Obviously, this cannot ensure that the gradient coils will meet the standards; furthermore, even if a problem is found in the resistance and inductance during the routine tests, since the direct gradient field data are not available, a further analysis cannot be made to provide corresponding solutions.
In order to obtain the direct gradient field data during the research and development stage of a gradient coil, some manufacturers had a gradient coil positioned in a testable main magnetic field (for example, the main magnetic field generated by the magnet of a sub-resonance imaging system), and applied a small direct current to the gradient coil, so that it was possible to carry out tests by simply using magnetic field testing tools currently available. By subtracting the magnetic fields tested with the current on and off respectively, the situation of the magnetic field distribution generated by the gradient coil can be obtained indirectly. However, an additional magnetic field is needed in this test method (such as the testable main magnetic field mentioned above), and it is troublesome, time-consuming and laborious to install and dismantle a gradient coil in this additional magnetic field, therefore this method is only suitable for tests of limited times during the research and development stage, but not suitable for frequent tests during a manufacturing process.
Since an alternating magnetic field can be generated by applying an alternating current to a coil and then the magnetic field distribution can be obtained from the alternating magnetic field, the interference from the earth magnetic field can thereby be eliminated and the test accuracy can be improved. However, since the strength of the gradient field is close to that of the earth magnetic field, and the accuracy of tests by an ordinary detecting method and apparatus is relatively low, therefore some manufacturers have chosen to use probes of high sensitivity (such as magnetic flux gates and highly sensitive Hall probes, etc.) for carrying out gradient field tests. However, a problem brought inevitably by the highly sensitive probe is high cost, and so the choice is usually made to use only one probe, and multi-point measurements are performed by way of an accurate positioning apparatus, and usually about one hour is needed to complete a full test for a gradient coil. For this reason, such a method of using a highly sensitive probe is not only high in costs, but also low in test-efficiency and time-consuming.
Therefore, how to provide a simple, highly efficient method for testing the distribution of a magnetic field and an apparatus therefore has become a difficult problem in the manufacturing of coils.